In various reproduction systems, including xerographic printing, the control and registration of the position of imageable surfaces such as photoreceptor belts, intermediate transfer belts (if utilized), and/or images thereon, is a critical feature. It is well known to provide various single and/or dual axes control systems, for adjusting or correcting the lateral position and/or process position or timing of a photoreceptor belt or other image bearing member of a reproduction apparatus, such as by belt lateral steering systems and/or belt drive motor controls, and/or adjusting or correcting the lateral position and/or process position or timing of the placing of images on the belt with adjustable image generators such as laser beam scanners.
Color registration systems are often implemented as a part of an overall color printing system. One example of a color registration process and/or system is disclosed in U.S. Pat. No. 6,275,244 entitled “Color Printing Image Bearing Member Color Registration System,” which issued to Omelchenko, et al on Aug. 14, 2001 and is assigned to the Xerox Corporation of Stamford Conn. U.S. Pat. No. 6,275,244 is incorporated herein by reference. Another example of a color registration system/process is disclosed in U.S. Pat. No. 6,300,968 entitled “Color Printing Process Direction Color Registration System With Expanded Chevrons,” which issued to Kerxhalli et al on Oct. 9, 2001 and which is assigned to the Xerox Corporation of Stamford, Conn. U.S. Pat. No. 6,300,968 is incorporated herein by reference. An example of a color printing or rendering process and/or system is disclosed in U.S. Pat. No. 7,039,348, entitled “Method for Maintaining Image on Image and Image on Paper Registration,” which issued to Kerxhalli et al on May 2, 2006 and which is assigned to the Xerox Corporation of Stamford, Conn. U.S. Pat. No. 7,039,348 is also incorporated herein by reference.
An important application of such accurate image position or registration systems is to accurately control the positions of different colors being printed on the same intermediate or final image substrate, to insure the positional accuracy (adjacency and/or overlapping) of the various colors being printed. That is not limited to xerographic printing systems. For example, precise registration control may be required over different ink jet printing heads and/or vacuum belt or other sheet transports in a plural color ink jet printer.
It is well known to provide image registration systems for the correct and accurate alignment, relative to one another, on both axes, of different plural color images on an initial imaging bearing surface member such as (but not limited to) a photoreceptor belt of a xerographic color printer. That is, to improve the registration accuracy of such plural color images relative to one another and/or to the image bearing member, so that the different color images may be correctly and precisely positioned relative to one another and/or superposed and combined for a composite or full color image, to provide for customer-acceptable color printing on a final image substrate such as a sheet of paper. The individual primary color images to be combined for a mixed or full color image are often referred to as the color separations.
Known means for adjusting the registration of images on either or both axes (the lateral axis and/or the process direction axis) relative to the image bearing surface and one another include adjusting the position or timing of the images being formed on the image bearing surface. That may be done by control of ROS (raster output scanner) laser beams or other known latent or visible image forming systems.
In particular, it is known to provide such imaging registration systems by means of MOB (“Marks-on-Belt”) systems, in which edge areas of the image bearing belt (either process or lateral direction, as described herein) outside of its normal imaging area are marked with registration positional marks, detectable by an optical sensor. For belt steering and motion registration systems (as previously described) such registration marks can be permanent, such as by silk screen printing or otherwise permanent marks on the belt, such as belt apertures, which may be readily optically detectable. However, for image position control relative to other images on the belt, or the belt position, especially for color printing, typically these registration marks are not permanent marks. Typically they are distinctive marks imaged with, and adjacent to, the respective image, and developed with the same toner or other developer material as is being used to develop the associated image, in positions corresponding to, but outside of, the image position. Such as putting the marks along the side of the image position or in the inter-image zone between the images for two consecutive prints. Such marks-on-belt (MOB) image position or registration indicia are thus typically repeatedly developed and erased in each rotation of the photoreceptor belt. It is normally undesirable, of course, for such registration marks to appear on the final prints (on the final image substrate).
It is generally well known in the art of reproduction systems that image registration control on an image bearing belt can be accomplished based on MOB sensor measurements of developed marks on the belt indicative of respective image positions on that image bearing member (substrate). If desired, that can also be combined with additional sensor information from belt edge sensing and/or permanent belt marks or holes sensing. As also noted, a printer image registration controller and/or electronic front end (EFE) can utilize MOB sensor inputs to control ROS scan lines positioning on the photoreceptor (PR) surface to correct registration of the respective image positions on both axes. That is, without necessarily requiring MOB sensor interaction with, or control over, the PR drive or PR steering controls for process direction or lateral direction registration. However, such PR registration movement, instead of, or in addition to, such imaging position registration movement, can also be accomplished if desired.
A direct sensing of the surface motion of image receivers, such as photoreceptor belts or intermediate transfer belts, or other substrates, enables more precise transport and/or image registration, for superior image quality. In contrast, a principle method of accurately sensing photoreceptor belt motion in the process direction in present practice for xerographic printers is to use a relatively expensive precision machined encoder roll in contact with the back surface of the belt (or on the drive shaft of the belt drive). The encoder roll may be coupled to a rotary shaft encoder with an anti-rotational coupling that helps avoid errors from misalignment. Motion sensing errors that can contribute to errors in color registration systems with such belt-driven encoder sensors can come from encoder roll eccentricity, rotary encoder accuracy (once per roll revolution errors) as well as errors from belt slip, belt stretch, and belt thickness variations. The once per encoder roll revolution type errors may be addressed by employing design rules that locate marking elements spaced apart at integer numbers of encoder roll revolutions, to synchronize the errors between color separations. However, this imposes disadvantageous machine architecture and physical size constraints. Avoiding those constraints could enable smaller size/height machines, or location of multiple image stations on the same side of a photoreceptor belt module, which in turn enables avoidance of other errors that can be encountered when locating imagers on both sides of a belt module.
Low frequency process direction movement errors, such as once per belt revolution, or other errors that accumulate in transporting the belt for multiple images, may be invisible to encoder roll registration controls. This is primarily attributed to belt thickness variations caused by the encoder roll sensing the belt from the backside with the belt flowing over this roll in a wrap condition. Advantages of the subject position measurement system include eliminating such error sources and thus improving registration. By directly measuring the belt surface position with a high degree of accuracy, those sensor signals can be inputted into an agile beam imager, such as the variable imaging position ROS systems shown in FIG. 1 to implement a printing system that can allow relaxation of motion control requirements or tolerances for the belt surface, and potentially eliminating the need for an expensive precision belt movement rotary encoder and its circuitry.
Color registration systems for printing, as here, should not be confused with various color correction or calibration systems, involving various color space systems, conversions, or values, such as color intensity, density, hue, saturation, luminance, chrominance, or the like, as to which respective colors may be controlled or adjusted. Color registration systems, such as that discussed herein, relate to positional information and positional correction (shifting respective color images laterally or in the process direction and/or providing image rotation and/or image magnification) so that different colors may be accurately superposed or interposed for customer-acceptable full color or intermixed color or accurately adjacent color printed images. The human eye is particularly sensitive to small printed color mis-registrations of one color relative to one another in superposed or closely adjacent images, which can cause highly visible color printing defects such as color bleeds, non-trappings (white spaces between colors), halos, ghost images, etc.
Image registration in the context of rendering systems, such as color printers, requires ensembles of chevron marks to be written on a photoreceptor or intermediate belt. These chevron ensembles can be detected by the MOB sensors described above. In order to correctly process the chevron ensemble data, image registration controller software is employed, which expects the chevron ensembles to contain a fixed number of marks in a set color order.
In many xerographic printers, low toner throughout can cause significant aging of the materials in the developer housing because the housing is churning the same material repetitively. The desired state is to disable developer housings that are not being used in order to preserve the materials. In this case, an image registration system cannot measure color-to-color registration properly because it expects a specific pattern of chevron marks.